Luminescent coating for electric lamps



Oct. 19, 1937. J. 30x 2,096,693

LUMINESCENT COATING FOR ELECTRIC LAMPS Filed April 3, 1937 IN VENTOR:

JAM/5s L. Cox.-

ATTORNEY.

Patented Oct. 19, 1937 UNITED STATES.

nummnscnn'r comma iron mnc'rmc r 1 ms 1.. Cox, Ma... minor to Hye Sylvania Corporation, alem, Mala, a c rporation of Massachusetts rr mien sun 3, maserm No. 134.5

scams. (or. 176-122 Luminescent material has heretofore been ap-' plied to the inner surface of lamp bulbs ortubes by first coating the surface with a glycerine binder or with a transparent enamel, then dusting on the luminescent material, thereafter heating the glass to the softening point to remove the volatile parts of the binder and to cause the luminescent particles to sinkinto the glass surface, adhering thereto. Such a process tends to produce an unevenly luminescent surface,'-because of the difficulty of the dusting operation. Such dusting is especially difficult, if not practically impossible, in the case of glass tubing bent to the complicated patterns used in neon signs. In such cases the coating of the tube must be done before the tubing is shaped, which is not always desirable. Furthermore, the luminescent coating has a short life.

5 The object of the present invention is to provide a more firmly-adherent coating which is uniformly and brightly luminescent, and which is easily applied. The methods used heretofore required dusting of the luminescent material on the surface of a binder. The material could not be mixed with the binder and applied directly, because a uniform coating could not be produced and because the 'binders were not transparent to ultraviolet radiation, 'up-n the action of which such materials usually depend. Furthermore, the particle size of the luminescent material is important in determining the brightness, and the size would usually be incorrect with the binders The new and novel process of the present invention greatly simplifies the application of luminescent coatings on the interior of a lamp bulb or tube. The luminescent material is mixed with a novel binder, producing a mixture of syrup-like consistency which can be directly applied to the inner surface of the glass. This mixture is poured over the inner surface to be coated, shaken around if desired and the excess drained off. If desired, the article to be coated so may be arranged with its longitudinal axis ver-' tical, the novel mixture being poured over its surface at the top and allowed to drain downward, thus covering the entire surface. Be-- cause of the viscosity of the mixture, the rate 55 of draining is slow. The coating produced in such a manner will not show streaks nor uneven- I When the excess has-drainediofl', and the coating is fairly dry, the bulbor tube is heated to a temperature near the softening point' ofthe glass. At first, the coating will turn black, due to the carbonization of the" binding material. As the heating is continued'in the presence of oxygemtor of air, the carbon will be oxidized and lost as a gas,leaving only the luminescent-:material in" a firmlyadhering film on the glass, or partially'im'beddedin the glass. 7

The drawing shows .a side view, partly in secti'on, of a lamp with an internal coating according to the invention. 7 e

Referring to the drawing the-lamp comprises a container l havingelectrodes {and sealed into the ends-thereof. These electrodesmay be thermionic if desired. The container has the porous layer 4 (indicated bythe broken lines in the figure) of luminescent material on itsinner surface. The container will usually comprise a gaseous'filling, at any desired pressure, of an inert gas such as argon and a quantity of mercury. The radiation from the discharge strikes the luminescent surface, causing the latter to give of! visible light.

For providing thefluorescent coating a mixture of binder and fluorescent material which is desirable for my process can be made as follows: 4 grams of dry nitrocellulose of about 1000 secwill not adhere properly to the glass, so a plastic sizing agent, such as n-butyl phthalate should be added to the acetate-nitrocellulose mixture, preferably before the luminescent material is added. A few drops of the n-butyl phthalate will generally be suflicient. The proportions given above can be varied considerably without afiecting the invention. Air bubbles which. tend to form in the mixture can be removed by placing the mixture for a time in vacuum before applying to the glass. These bubbles will tend to spoil the finished surface, if not removed. They may also be removed, without the evacuation, by pouring the mixture in a fine streamon th surface to be coated.

The'nitrocellulose used must be of high viscosity in order to keep the particles of luminescent material in suspension long enough for 2" thecoatingtobe applied. Low mew binders tend tov separate from the luminescent material while the coating is being applied.

It will be noted from the foregoing description ofmyprocessthatniycoatingisinitiallyasimwhich the fluorescent materials provide.

pensionof luminescent material in a'high' vii-'- cosity binder. This is applied'to the desired surface-and dried. The surface then contains a layer, or preferably several particles, with the binder poses the binder, leaving a carbon residue in the interstices between luminescent particles. 'Ihe next step, further heatinl'. preferably to a tem- 'perature higher than that of the first step. in an atmosphere containing oxygen, oxidlses the carbon to a gaseous compound causing it to disappear from the interstices of the luminescent coating. 1 thus obtain a pcrouscoating, preferably'sev'ersl layers thick, of luminescent material. The porosity produces a greater brightness of the luminescent surface. because it enables full utilization *of the exciting light to produce lunflnescence. The exciting light which penetrates one layer will be absorbed by the next.

Any visible light emitted by luminescence in the interstices will reach the surface, by a reflection from the good reflecting surface for visible light The number of layers should be enough to utilizeas much as possible the exciting light without losing too much visible emission by the absorption occurring at the successive reflections.

My novel process is seen to differ sharply from the methods known'in the art,. methods which first coat the surface with-a volatile binding material, then dust on fluorescent material thereafter removing the volatile material by heating. Such a process does not immerse the fluorescent material in the binder, and does not pass through a stage in which the luminescent coating conlayers, of luminescent filling the interstices between particles. The first heating step decomtains particles of carbon which can be removed by oxidation to leave a porous coating.

"lhecharacterofthesurfaceproducedbymy coating thus difiers sharply from that product by methods heretofore known. y virtue of the porqs ty iust described.

1 -'Ihe viscosity, for the nitrocellulose, given in seconds'is the time required. in the given liquid;

for a standard steel ball to drop 10 inches.

A fluorescent of phosphorescent material may be considered as a luminescent material for the pin-pose of this invention. Typical materials are calcium tungstate, zinc sulphide, zinc-silicate, zinc-cadmium sulphide and magnesium molybdate, although any other luminescent material may be used. 'If desired. of course, several such materials may be used together.

What I claim, and desire to secure by Letters Patent, is:

l. A process for applyingluminescent material the interior of the, container of an electric lamp devicejwhich comprises the steps of coating the inner surface of said container with a suspension of luminescent material in a high viscosity binder, heating until'the binder turns brown. and then heating in the presence of oxygen until this brown color disappears.

2. A process for applying" luminescent material to the interior of the container of an electric lamp devicewhich comprises the steps of coating the inner surface of said container with a sus- 

